1. Field of the Invention
The invention generally relates to magnetic resonance imaging techniques. More particularly, the invention relates to magnetic resonance imaging techniques useful in situations in which gross transient motion and/or magnetic field inhomogeneities are present within the field of view.
2. Summary of the Prior Art
Magnetic resonance imaging is the optimal imaging modality for surgical applications primarily because of its ability to elucidate a wide variety of lesions. Stereotactic systems employing magnetic resonance imaging steadily have been enhanced, thereby improving a surgeon's ability to safely and efficaciously operate. The latest step in this evolution has been the ability to provide intraoperative updates of previously obtained magnetic resonance images of tissues of interest in a manner that allows the surgeon to track changes therein throughout the course of a particular surgical procedure.
Unfortunately, the phase sensitivity of conventional two and three-dimensional Fourier transform magnetic resonance image scans to motion and varying magnetic field inhomogeneities causes problems. Specifically, it has been found that when there is motion within the imaging field of view during such conventional magnetic resonance imaging, undesirable artifacts are created in the resulting image. Similarly, varying magnetic field inhomogeneities in the imaging field of view also cause undesired artifacts in the resulting images. Accordingly, it presently is necessary to halt the surgical procedure, and to clear the surgical field of all metal operating instruments and the like, each time a conventional Fourier type magnetic resonance image is to be taken. Obviously, this is not a satisfactory requirement.
Besides multi-shot phase encoded, two-dimensional or three-dimensional Fourier imaging, the technique of so-called "line-scan imaging" also is known in the art. Generally speaking, line scan imaging sequentially acquires the individual lines of a magnetic resonance image in multiple independent single shots. Phase encoding is not used in line-scan imaging. Instead, only the magnitude of the Fourier-transformed signal is used. Known and established single-shot techniques, by their nature, do not have shot-to-shot phase encoding, although they generally have phase encoding within each shot. Therefore, a line-scan imaging sequence is immune to the phase encoding errors mentioned above that are encountered in conventional Fourier magnetic resonance imaging methods.
Typically, however, line-scan generated image signals demonstrate smaller signal-to-noise ratios than are present in conventional imaging techniques. This results in less distinct resultant images. In addition, line-scan imaging basically eliminates ghosting artifacts caused by gross transient motion-related shot-to-shot phase variations. Nevertheless, signal losses within each of the isolated lines still may occur due to non-uniform motion and/or magnetic field phenomena during the acquisition of the individual lines.
Accordingly, a technique for magnetic resonance imaging useful during the course of a surgical procedure or the like, without the requirement of a cessation of the procedure and/or a clearing of the field of view to be imaged, would be a significant advance in, and beneficial to, the art.